The present invention relates to an image reading apparatus having a plurality of line sensors, and more particularly, to a sensor-to-sensor distance correcting method for correcting a registration error of data read at a specific point of time due to a difference in spatial disposition intervals between the line sensors in a focusing optical system constituted by the plurality of line sensors, and a circuit therefor.
In general, a document image reading apparatus such as an image scanner, a digital copier or a facsimile, reads an image using a linear image sensor which arranges light sensing elements one-dimensionally. These image reading apparatuses have been developed such that its resolution and coloration have improved image quality, and the speed thereof has been increased to enhance their performances. Operations of these image reading apparatuses will be described referring to FIGS. 1A, 1B and 2.
Image reading apparatuses employing a reduction-type focusing optical system and an equivalent magnification-type focusing optical system shown in FIGS. 1A and 1B, respectively, read an image through a main-scan by radiating a light source to one-line image on a manuscript and focusing a reflective light on a linear sensor. As shown in FIG. 2, a two-dimensional manuscript image is input through a sub-scan wherein a line image is read by transferring the linear sensor to a relative next-line position based on the manuscript. At this time, an optical apparatus such as a lens is required between the manuscript and the linear sensor to optically adjust the widths of the manuscript and the linear sensor. In general, when the width of the manuscript is less than that of the linear sensor, the optical apparatus is referred to as a reduction optical system (FIG. 1A), and when the width of the manuscript is the same as that of the linear sensor, it is called an equal-magnification optical system (FIG. 1B). The image reading apparatus is classified into two focusing methods.
The resolution of the image reading apparatus is determined with respect to each of the main scan and sub-scan directions. Thus, the resolution of the main scan direction is determined by the number of light sensing elements in the linear sensor and a reduction ratio of an optical system. Also, the resolution of the sub-scan direction is determined by the distance by which the position of the image sensor is moved relative to the manuscript image.
In order to accomplish a high resolution in the main scan direction, there are methods of: (1) increasing the number of light sensing elements of the sensor corresponding to a unit length on the manuscript by improving the integration density of the sensor while the reduction ratio of the optical system is maintained; and (2) focusing an image by increasing the number of light sensing elements and decreasing the reduction ratio without a change in the size of each light receiving element. In the former method, since a light receiving area of each light sensing element on which a unit pixel on the manuscript is focused is made narrow, the sensitivity of the light sensing element must be improved significantly, the light amount of the irradiated light source must be increased, or a light sensing time (or an exposure time) must be prolonged. In this method, the total number of light sensing elements must be increased by three times in red (R), green (G) and blue (B) according to the coloration, which in practice is not possible. Meanwhile, in the latter method, though the length of the sensor is greatly increased to thus exceed the diameter of an existing semiconductor wafer, there is a problem in fabricating the sensor and in the yield thereof.
Thus, in order to overcome the above problems and accomplish a colorful, high resolution and high speed image reading apparatus, in the case of the reduction-type focusing optical system, sensors are used being isolated in a sub-scan direction by differing the positions of the red, green and blue R, G and B according to a photosensitive wavelength band, as shown in FIG. 3A. Also, in the case of the equal magnification-type focusing optical system, the sensors are divided into a proper size and processed, and then disposed in a zigzagging manner as shown in FIG. 3B. Such an alternative measure has been widely employed since it is very realistic and economical enough to extensively apply a technology accumulated in a conventional black and white image reading apparatus and an associated constituent technique. However, in this method, the respective sensors are disposed in a zigzagging manner with predetermined distances based on the manuscript such that the geometrical positions of images read by the sensors at a specific moment are different from each other, whereby a correction via a proper method is required.
In general, the interval between the sensors maintains the relationship between a multiple of an integer of the width and the length of each light receiving element which are determined by a resolution in the main scan direction provided by the sensors. Accordingly, a reading registration error on the manuscript due to the intervals between the sensors can be corrected by a relatively simple means by corresponding the reading registration error to a multiple of an integer of sub-scan line. That is, when the resolution in the main scan direction equals that in the sub-scan direction, or when the resolution in the sub-scan direction is a multiple of an integer of that in the main scan direction, a registration correction due to the interval difference between the sensors is performed in a sub-scan line unit. A method for simply correcting the registration error using a line delay memory is already known. However, this method is not flexible in an actual case. In a document image reading apparatus, the resolutions in the main scan and sub-scan directions are arbitrarily and finely adjusted so that various magnification conversions are possible according to the relationship with an output apparatus. Accordingly, since the reading registration error in the sub-scan direction due to the intervals between the sensors is not always correctly expressed as an integral relation in a sub-scan line depending on an arbitrary magnification conversion, registration occurs to a certain extent so that an additional correcting means is necessary.
As described above, means for adaptively correcting a registration error in accordance with the arbitrary magnification conversion in the main scan direction actually becomes very essential and necessary for obtaining a high quality image.
Meanwhile, as shown in FIG. 4, the U.S. patent applications Ser. Nos. 4,926,041, 5,019,703, 5,113,067, 5,173,599 and 5,361,145 propose a method for correcting geometrical distortion due to the interval difference between the sensors by using dichroic filters (or blazed diffraction mirrors or beam splitters) having different refractive indices as reflective mirrors to focus a line image at an identical position on a manuscript to each sensor. However, the above proposed method cannot be applied to the case in which each sensor is disposed in a zigzagging manner in the configuration of the equal magnification optical system as shown in FIG. 1B. Also, since the above method requires great care and elaborateness in a planarization degree, the thickness and an assembling angle for fabrication and assembly of the reflective mirror as a dichroic means in order to perform an accurate correction, the reflective mirror is not an easy means.
Meanwhile, as shown in FIG. 5, the U.S. Pat. No. 4,953,014 is applicable to all of the sensors shown in FIGS. 1A and 1B. However, since the interpolator is a linear interpolator constituted only by a multiplier and an adder as shown in FIG. 6, it cannot avoid an aliasing phenomenon and a jagged error on a delicate image such as a star target shape, thereby degrading image quality. That is, a limit due to the linear interpolation is inevitable. Furthermore, the structure of the delay memory is incomplete since it does not provide any propositions about an appropriate method for optimizing a memory capacitance and operating the memory at a high speed.